[NET]: Corrects a bug in ip_rt_acct_read()

It seems that stats of cpu 0 are counted twice, since
for_each_possible_cpu() is looping on all possible cpus, including 0

Before percpu conversion of ip_rt_acct, we should also remove the
assumption that CPU 0 is online (or even possible)

Signed-off-by: Eric Dumazet <dada1@cosmosbay.com>
Signed-off-by: David S. Miller <davem@davemloft.net>